The invention relates to a thermal infrared sensor in a housing with optics and a chip with thermoelements in the form of thermopile, bolometer or pyroelectric sensor structures on a membrane, which spans a frame-shaped support body that is a good heat conductor, wherein the support body has vertical or approximately vertical walls.
There exist infrared sensors that can be produced in a variety of variants using silicon micromachining technology.
For example, US 2003/0054179 A1 introduces a sensor cell for, inter alia, thermopiles, with a thin membrane and an absorber affixed thereon.
The support substrate beneath the sensor structure is mechanically recessed by means of a wet chemical etching process of surface micromachining. In this etching process slits are etched into the membrane. This wet etching process produces inclined walls. In no case does the depth of the clearance reach the complete depth of the substrate. That means that the smaller the sensor structure in its lateral dimensions, the smaller the etching depth.
If a high vacuum is not used, then the thermal conductivity of the residual gas or the fill gas in the sensor housing reduces the temperature differential that can be attained between the absorber region—that is, the “warm contacts”—and the heat sink—that is, the support substrate.
If the absorbed IR radiation produces a smaller temperature differential, then the sensitivity that the sensor cell can attain is also reduced. The use of a fill gas is not described.
Kanno, T. et al. (NEC Corp.): “Uncooled focal plane array having 128×128 thermopile detector elements” in B. Andersen (editor), Infrared Technology, Proc. SPIE 2269, Vol. XX, San Diego, July 1994, pp. 450-459 describe a monolithic thermopile sensor array, in which the sensor elements are produced by surface micromachining technology with a sacrificial coating. The distance between the sensor structure and the heat sink in the substrate is significantly less than the substrate thickness itself. The solution permits a relatively good resolution capacity only for the case that the sensor chip is capped in the high vacuum sealed housing. In contrast, the cost effective housing structures under low residual gas pressure or with a fill gas do not produce an adequately high sensitivity.
Both DE 693 29 708 T2 and EP 0 599 364 B1 relate to a fabrication process for infrared radiation sensors, wherein the sensitivity is enhanced through the use of a vacuum housing or a housing that is filled with a gas that is only slightly heat conductive.
The engineering object of this fabrication method is to be able to provide a vacuum or a gas exchange without damaging the membrane, on which is located the sensor element, due to the pressure differentials. In this case the crucial factor is that the gas exchange in the cavity below the membrane is approximately 300 torr as the burst pressure differential for the silicon oxide/silicon nitride membranes that are used.
In addition to recesses in the bottom plate and spacers of different shapes, there are also apertures in the membrane as ventilation methods.
All of the drawings show wet etched, inclined trench walls. Between the base plate and the substrate there is a ventilation gap, which serves preferably to compensate for the pressure between the region above and below the membrane.
The proposed solution exhibits slits in the membrane that, however, do not permit optimal use of the available area, because such utilization is prevented by the illustrated etching process with the inclined walls. The influence of a slitted membrane on the thermal conditions in the chip is not taken into consideration.
The patent DE 102 60 408 A1 describes a thermal radiation sensor array comprising a slitted thermopile array. The basis is the stacking of a number of thermopile arrays one above the other. The slits are not used to enhance the sensitivity, but rather so that the sub-arrays can always be “seen” through the slits of the array lying above.
The intent is to increase the resolution capacity of the system in this way. However, the described arrangement exhibits a plethora of drawbacks. The process costs rise by a factor of the number of stacked linear arrays that are used; similarly the exact alignment of the individual chips that are stacked one above the other and their connection to each other also means a high technological complexity and financial outlay, and the pixels can be used only to achieve a linear array, because they are not square and are, thus, unsuited for generating a two-dimensional infrared image. Furthermore, the membrane is structured by rear sided, anisotropic wet etching, a technique that results in a considerable enlargement of the sensor structure owing to the inclined walls of the heat sink. Only the slits are dry etched by reactive ion etching (RIE). The aforementioned pairing of materials for the thermopiles (bismuth/antimony) and the absorber material (silver black coating) are barely compatible with the CMOS process. Another drawback is the high temperature dependence of the sensitivity of the aforementioned material pairing. It must also be added that the targeted multiplication of the resolution capacity is always limited by the optical system, because the individual sub-arrays lie in different sharpness planes and, therefore, the only alternative is a compromise in the choice of the focal distance on the image side. A gas filling and/or a housing surrounding the sensor cells with reduced inner pressure in order to enhance the sensitivity owing to the reduced thermal conductivity of the gas, is not described.
A monolithic thermopile sensor array, which is produced by bulk Si micromachining technology, is described in the HORIBA product information: “8×8 element thermopile imager,” in Tech Jam International, Sep. 26, 2002. The 64 elements are situated on an 8×8 mm chip, each element being separated thermally by Si walls by the wet etching technology. The size of the chip as a function of the process leads to relatively high manufacturing costs and again stands in the way of cost effective mass applications.
In addition to these thermopile solutions, there are other solutions for low cost infrared arrays.
U.S. Pat. No. 4,472,239 A and U.S. Pat. No. 4,654,622 introduce thermal sensor structures with a thin membrane and slits for etching out parts of the support substrate lying underneath. In both cases the underneath recesses reach only a shallow depth, a state that allows, as in the case of the solutions described above, just a slight sensitivity with simultaneously cost effective housing solutions without high vacuum sealing.
DE 199 54 091 A1 and U.S. Pat. No. 6,342,667 describe thermopile sensor cells, in which the recess beneath the sensor structure is freely etched by means of slit structures in the form of large triangles in the edge region of the membrane or in the form of a cross in the center of the membrane. In both cases the results are produced by a wet etching process that does not permit the occurrence of large distances from the heat sink owing to the inclined walls. A plurality of parallel arranged thermoelements prevents large temperature differentials between the hot and cold contacts and, thus, a high signal sensitivity cannot be obtained.
DE 198 43 984 A1 introduces cells of infrared radiation sensors. The embodiment in FIG. 1 has vertical walls that run through the entire substrate. However, it also describes a plurality of more likely “short” thermoelements that do not—as stated above—permit a high sensitivity. The other exemplary embodiments in FIG. 2 e) and FIG. 3 a) introduce once more surface micromachining solutions for generating the recess, where the etching is performed through an aperture in the membrane. The depth of the etching can range from 50 to 200 μm. Again the short distance between the sensor structure and the heat sink of max. 200 μm does not enable high sensitivities owing to the heat conductivity of the gas. Possibilities for reducing the heat conductivity by means of gases of low conductivity or reduced pressure are not described.
EP 0 869 341 A1 introduces monolithic bolometer structures for an infrared sensor array. In the case of these bolometer structures the sensor elements are produced by means of surface micromachining technology, where the removal of a sacrificial coating results in sensor bridges that exhibit very good thermal insulation at approximately 2.5 μm above the Si substrate that contains the evaluation circuit.
In the meantime such infrared bolometers with sensor bridges have become available in many variants. Because they permit very small element dimensions, they are widespread in high-resolution infrared arrays. In principle, despite the small sensor element dimensions, very good temperature resolutions are achieved with this method.
However, the small element dimensions on the silicon surface automatically require high vacuum sealed packaging of the sensor chip, a feature that once again is at variance with cost effective mass production. The possibility of sealing this chip in a cost effective housing with gases of low thermal conductance under normal pressure conditions would drastically reduce the attainable sensitivity.
DE 40 91 364 C1 introduces a thermopile sensor cell with a thin membrane and slitted structure. The absorber region is held via a long beam and a few thermoelements. The membrane can have holes or slits. The beam that has the thermoelements and that can exhibit a width of 130 μm is insulated from the substrate edge and the absorber region by means of slits that are also quite wide.
The support substrate that lies beneath the sensor structure is wet etched from the rear side—with suitably inclined walls in the substrate. A filling operation with protective gas is provided.
In principle, higher temperature differentials and sensitivities can be achieved with such a solution. However, the wide slits prevent optimal utilization of the available area (degree of filling) of the sensor cell. The wet etched recess in the support substrate exhibits walls that run towards the outside; the whole sensor cell is supposed to be about 2×2 mm in size. The inclined substrate walls that run towards the outside do not permit any small sensor cells or short distances between the cells (array structures).
Working on this basis, the prior art solutions propose thermal infrared sensor cells that do not permit a cost effective production of thermal infrared sensor cells exhibiting a high sensitivity and simultaneously a high degree of filling and simple housing technology without a high vacuum sealed housing closure either for reasons relating to the large area chip technology or too little distance between the sensor structure (absorber region) and the support substrate located underneath or for reasons relating to the costly vacuum housing technology.